1. Field of the Invention
The present invention relates to a radio communication apparatus and a power consumption control method therefor and, more particularly, to a radio communication apparatus used as a mobile station in a mobile communication system and used in a CDMA communication system to receive signals from a plurality of CDMA transmitters as in handover operation, and a power consumption control method therefor.
2. Description of the Prior Art
Recently, mobile communication systems such as a system using portable telephones have become widespread. One of the communication schemes used by such mobile communication systems is CDMA (Code Division Multiple Access).
According to CDMA, on the transmitting side, data is spread by using one of predetermined spreading codes which differ depending on the data to be transmitted, and the spread data is transmitted. On the receiving side, the data is obtained by spreading (so-called despreading) the reception signal by using a spreading code identical to the one used on the transmitting side (to be precise, a code complex conjugate to the spreading code on the transmitting side). In such communication based on CDMA, the peak correlation value of a signal received on the receiving side is found out by shifting the despreading timing, thereby regenerating the signal transmitted from the transmitting side.
Owing to demands for high portability and the like, mobile stations in a mobile communication system using portable telephones or the like are required to be smaller and lighter and to be used for a long period of time. For this reason, built-in batteries for driving mobile stations are also reduced in size. In order to allow mobile stations to operate on such compact batteries for a long period of time, it is important to reduce the power consumption of the mobile stations.
The CDMA receiver disclosed in Japanese Unexamined Patent Publication No. 9-200177 is as an example of a technique of reducing the power consumption of a CDMA receiver. This CDMA receiver is designed to reduce the power consumption of a correlation filter for despreading a reception signal.
FIG. 1 is a block diagram showing a correlation filter used in the CDMA receiver disclosed in Japanese Unexamined Patent Publication No. 9-200177.
As shown in FIG. 1, this correlation filter is comprised of a delay circuit 51, weighting/combining circuit 52, timing control circuit 53, and switch element 54.
The delay circuit 51 is used to delay an input signal. When an input signal is a digital signal, the delay circuit 51 is formed by, for example, a shift register. When an input signal is analog signal, the delay circuit 51 is formed by, for example, a delay line or analog shift register.
The weighting/combining circuit 52 is prepared in correspondence with tap outputs TP1 to TPn and comprised of a plurality of weighting circuits for multiplying the tap outputs TP1 to TPn by weighting factors W1 to Wn and a combining circuit for combining output signals from the respective weighting circuits and outputting the resultant signal.
The timing control circuit 53 controls the operation of the switch element 54 on the basis of a correlation output signal obtained from the weighting/combining circuit 52. The switch element 54 is kept closed for a predetermined period of time in accordance with a control signal from the timing control circuit 53 so as to apply a power supply voltage from a power supply 55 to the weighting/combining circuit 52.
The weighting/combining operation of the weighting/combining circuit 52 is equivalent to detecting the correlations between the input signal and weighting factors W1 to Wn. In the CDMA receiver disclosed in Japanese Unexamined Patent Publication No. 9-200177, since the correlation results do not exhibit peaks in all parts of the input signal, the supply of power to the weighting/combining circuit 52 is stopped during periods corresponding to parts other than the parts in which the correlation results exhibit peaks, thereby reducing power consumption.
In the CDMA mobile communication system, when a mobile station moves from the cell covered by a given base station to the cell covered by another base station, the mobile station performs a handover, i.e., switching to another base station from which signals are to be received, after a period of time during which signals from two base stations are received.
For example, in this handover operation, the mobile station must receive signals from a plurality of base stations and demodulate the signals. This increases power consumption.
According to a conventional technique like the CDMA receiver disclosed in Japanese Unexamined Patent Publication No. 9-200177, however, no consideration is given to a reduction in power consumption in handover operation, and hence a large amount of power is still consumed in handover operation.
In the conventional technique, no consideration is given to a reduction in power consumption in an arrangement in which a CDMA receiver receives signals from a plurality of CDMA transmitters, i.e., a site diversity arrangement or the like, in addition to a reduction in power consumption in handover operation.